1. Field of the Invention
The present invention relates to a power module with a mounted semiconductor chip, and a method of manufacturing the same.
2. Description of the Prior Art
Recently, with a demand for high performance and miniaturization of electronic equipment, smaller power source circuits consuming less power have been increasingly desired in the power electronics field. The heat dissipating structure of a power module is a key issue for the purposes.
In a method proposed to improve heat dissipation of a power module, a substrate with high thermal conductivity is used. A semiconductor chip is mounted on this substrate so that the backside (the side opposite to the device-forming surface) of the semiconductor chip contacts with the surface of the substrate, and heat is dissipated from the substrate.
To achieve this purpose, for example, a method using a metal-base substrate has been proposed. The metal-base substrate has a metal sheet such as aluminum, an insulating layer formed on the metal sheet, and a wiring pattern formed on the insulating layer. Another method employs a substrate prepared by joining a copper sheet to a surface of a ceramic sheet such as aluminum oxide and aluminum nitride.
In another proposed method, a lead is formed on a surface of a substrate prepared by dispersing an inorganic filler in a thermoplastic resin, and a semiconductor chip is mounted thereon. However, since this substrate is produced by melt-kneading a thermoplastic resin and an inorganic filler and injecting this mixture, the inorganic filler is difficult to fill with high density. As a result, the thermal conductivity cannot be improved remarkably.
In a method disclosed in JP-A-10-173097, a lead is formed on a surface of a substrate prepared by dispersing an inorganic filler in a thermosetting resin composition and a semiconductor chip is mounted thereon. This substrate is produced by laminating a sheet and a lead frame and subsequently by curing the sheet. The sheet comprises a thermosetting resin and an inorganic filler, and it is flexible in an uncured state. In this method, the inorganic filler can be filled with high density, and the thermal conductivity is improved.
In the field of signal circuits, flip-chip bonding or wire bonding is used for mounting semiconductor chips. Flip-chip bonding indicates mounting of a semiconductor chip face down so that the substrate surface and the semiconductor chip surface (device-forming surface) face each other. Flip-chip bonding provides mounting with higher density when compared with wire bonding.
However, in the field of power circuits, heat should be dissipated from the backside of a semiconductor chip. Therefore, in a conventional power module to dissipate heat from a semiconductor chip to a substrate, an improvement in heat dissipation cannot be expected when flip-chip bonding is used.
Therefore, wire bonding has been employed for conventional power modules. Wire bonding indicates that a semiconductor chip is borne on a substrate to contact the substrate surface and the backside of the semiconductor chip, and an electrode of the semiconductor chip and the lead formed on the substrate are connected with each other through a metal wire. However, since the conductive resistance of the metal wire is considerably great when compared with ON resistance of the semiconductor element, power loss is increased and so is the calorific value when a semiconductor chip is mounted by wire bonding. As a result, a substrate is required to have more heat dissipation capacity when wire bonding is used.